Prior to blockage of fish passage in the Columbia River Basin migration of anadromous fish from the Pacific Ocean would have brought with it large stores of marine derived nitrogen (N) and carbon (C) to central Idaho, USA. In a region dominated by nutrient poor soils and complex topography, anadromous fish may have been a major nutrient input to riparian forests. To examine the importance of this subsidy, anadromous fish carcasses and analog pellets (a carcass substitute) were placed within riparian forests where salmon runs have been blocked for nearly a century.
Results/Conclusions
Soil samples 0-10cm collected beneath these treatments revealed rapid development of nutrient “hot-spots”. Within 10 days of treatment, inorganic N (NH4+ + NO3-) significantly (p<0.05) increased from 2.2 mg kg-1 ± 0.3 SE to 1245 mg kg-1 ± 75 SE and dissolved organic carbon (DOC) significantly (p<0.05) increased from 205 mg kg-1 ± 27 SE to 510 mg kg-1 ± 30 SE. DOC and IN concentrations began to decline following snow melt and had returned to near background concentrations by one year post treatment. However, dissolved total N (DTN) as an indicator of organic N availability remained significantly (p<0.05) elevated by 250 mg kg-1 ± 125 SE. demonstrating the persistence of this nutrient subsidy beyond annual deposition events. Lack of elevated DOC and IN pools in conjunction with isotopic evidence of extensive vegetative N utilization attests to the nutrient poor status of these inland forests. Foliar isotope analysis of conifer seedlings found significant (p<0.05) enrichment in δ15N from 0.3‰ ± 0.06 SE pre-treatment to 4.7‰ ± 0.03 SE one year post treatment. Foliar %N did not significantly increase suggesting N uptake responsible for the shift in δ15N was quickly converted to additional biomass. Rapid utilization of N as well as the magnitude of soil C and N responses to these treatments demonstrates the vital nature of this nutrient subsidy to oligotrophic systems.